A scanning electron microscope is widely used as a tool in observing a biological sample and an organic sample at high resolution. Conventionally, when the biological sample and the organic sample are observed by the scanning electron microscope, in order to reduce electron beam damage to an observation target sample and obtain an image with high contrast, it has been considered essential to perform treatment for, for example, after fixing the sample with formaldehyde or the like, coating the surface of the sample with gold, platinum, carbon, or the like or applying dying by heavy metal to the sample.
However, in recent years, a method with which a biological sample can be observed at high contrast without coating and dying has been developed (see Patent Literature 1 and Non Patent Literature 1).
In this new method, a sample is deposited on a lower surface of a thin sample supporting film such as a carbon film and an electron beam with a low acceleration voltage is irradiated on the sample from above the sample supporting film. The irradiated electron beam spreads while diffusing on the inside of the sample supporting film and reaches near the lower surface of the film. Secondary electrons are emitted from the lower surface of the sample supporting film. The secondary electrons are absorbed by the sample immediately below the sample supporting film. Consequently, it is possible to obtain an image with extremely high contrast.
In this method, a condition is set such that energy of the secondary electrons is approximately 10 eV. With such extremely weak secondary electrons, electron beam damage to the observation target sample is markedly low. Therefore, even in a sample susceptible to damage such as a biological sample, an original shape and an original structure of the sample can be observed or analyzed with an image with high contrast. Such an observation condition is called “indirect secondary electron contrast condition”.
Such an observation method is further promoted to also develop a method of forming a conductive film below an insulative thin film layer and further improve resolution and contrast by making use of a charging effect by electron beam incidence (see Patent Literature 2 and Non Patent Literature 2).